It is known that silicon can be made in rod form by a process referred to as the Siemens process. A mixture comprising hydrogen and silane (SiH4) or a mixture comprising hydrogen and trichlorosilane is fed to a decomposition reactor containing, substrate rods which are kept at a temperature of more than 1000° C. Silicon is deposited on the substrate and by-product gas mixtures exit in a vent stream. When a mixture comprising hydrogen and trichlorosilane is used, the vent stream may include hydrogen, hydrogen chloride, chlorosilanes, silane, and silicon powder. For purposes of this application, the term ‘chlorosilanes’ refers to any silane species having one or more chlorine atoms bonded to silicon and includes, but is not limited to monochlorosilane (H3SiCl), dichlorosilane (H2SiCl2), trichlorosilane (HSiCl3), silicon tetrachloride (SiCl4), and various chlorinated disilanes such as hexachlorodisilane and pentachlorodisilane. In the vent stream, hydrogen and chlorosilanes such as silicon tetrachloride and trichlorosilane may be present both from un-reacted feed gas and reaction product from the decomposition. The vent stream is passed through a complex recovery process where condensations, scrubbing, absorption and adsorption are unit operations often used to facilitate the capture of feed material trichlorosilane and hydrogen for recycle. One problem associated with the Siemens process is that it is difficult to achieve a high yield of polycrystalline silicon product to silicon fed due to the chemical equilibria and kinetics that control this reaction process.
Quite often only 50%, or less, of the maximum theoretical yield of polycrystalline silicon is achieved.
An alternate process is to feed the mixture comprising hydrogen and silane or the mixture comprising hydrogen and trichlorosilane to a fluidized bed containing silicon nearly spherical beads that are maintained also at high temperature. The beads grow in size, and when large enough, are passed out the bottom of the fluidized bed reactor as product. The vent gases exit the top of the reactor and are sent through a recovery process similar to the one described above for the Siemens process. Yield in this system may be nearly 90% of theoretical maximum, as compared to the 50% for the Siemens process.
One problem with the fluidized bed reactor process is that one must heat the beads to a temperature higher than the average bed temperature to facilitate heat transfer. That can be done, for example, by use of hot walled reactor, microwave energy, or infrared radiation. All heating methods have unique operating problems. One problem, however, is that the bottom of the fluidized bed reactor may be hot, and the feed gas is reactive when it contains only trichlorosilane and hydrogen. As a result, the feed gas distributor, large beads, and reactor side walls are prone to rapid deposition of silicon. Those deposits subsequently disrupt the proper feed distribution, product separation, and heat transfer of the system. Another problem with the fluidized bed reactor process is the product quality is generally insufficient for use in integrated circuit manufacture; however, the product of the fluidized bed reactor process may be used in solar grade applications.
There is a need in the polycrystalline silicon industry to improve efficiency of polycrystalline silicon production and reduce by-products.